take a series of shots with around 30% overlap of each sequential image
ensure that you use the same exposure, white balance, zoom setting and focus for every shot
avoid lens vignetting - take care choosing your lens and aperture
avoid variances in sky colour by NOT using a polarising filter
keep the camera level whilst panning
avoid tilting camera up for tall objects as this will create converging lines
avoid sunlight hitting the front lens as this may cause different flare patterns on each image
consider having the camera in portrait mode (ie. held vertically) to gain even greater image quality but more images required
try to pan the camera around the nodal point of the lens, especially if there are foreground objects
if shooting manually shoot a black frame at start and at end to make it easier to locate the sequence later